Bone marrow transplantation is widely used with patients undergoing high dose chemotherapy or radiation therapy. The dose limiting side effects of chemotherapy and radiation therapy are their deleterious effects on hematopoietic cells through destruction of the bone marrow cells which are the precursor cells for all hematopoietic cells. This damage to the marrow results in myelosuppression or myeloablation, rendering patients susceptible to opportunistic infections for a prolonged period of time. Bone marrow transplantation involves the infusion of early bone marrow progenitor cells that have the ability to re-establish the patients' hematopoietic system, including the immune system. Transplantation decreases the time normally required for the restoration of the immune system after chemotherapy or radiation therapy and, thus, the time of risk for opportunistic infections.
Bone marrow cells contain totipotent stem cells which give rise to hematopoietic cells of all lineages including the lymphoid, myeloid and erythroid lineages. Stem cells have the ability to renew themselves as well as to differentiate into progenitor cells of all hematopoietic lineages. Progenitor cells retain the ability to proliferate and give rise to differentiated cells of all lineages. Differentiated cells lose the ability to proliferate and exhibit morphological characteristics specific for their lineages (such as macrophages, granulocytes, platelets, red blood cells, T cells and B cells). Stem cells and progenitor cells express CD34 on their surface while differentiated cells do not. Bone marrow includes stem cells as well as progenitor cells of the lymphoid (T and B cells), myeloid (granulocytes, macrophages) and erythroid (red blood cells) lineages.
For use in bone marrow transplants, hematopoietic precursor cells can be derived either from the cancer patient (autologous transplant) or from a histocompatible donor (allogeneic donor). These cells can be isolated from bone marrow, peripheral blood or from umbilical cord blood. In all cases, cells are harvested before chemotherapy or radiation therapy. The number of progenitor cells that can be harvested at one time is small and, in many cases, is not sufficient for a successful transplant. Accordingly, several methods have been developed to expand, in vitro, bone marrow cells or progenitor cells obtained from blood aphereses or from umbilical cord blood.
The ability to expand these cells has helped advance bone marrow transplant technology as a viable adjunct therapy for cancer treatments that involve high doses of chemotherapy and/or irradiation. However, the existing methods for hematopoietic cell expansion require the addition of appropriate cytokines to permit the in vitro expansion of hematopoietic stem cells. The high cost of such growth factors has adversely affected the ability of those skilled in the art to expand hematopoietic cells in vitro for transplantation or other purposes. Accordingly, a need exists to develop new methods for expanding hematopoietic cells in vitro which do not require exogenously added cytokines to support cell growth and differentiation.